[setopt]

I don’t know which field you work in, but I do research in physics and chemistry and L is the dominant convention in the papers I read. I also teach intro physics at the university level and the textbook uses L.

In high school (~2008) we were taught that this was the correct new abbreviation for liter in chemistry, but I remember that at the time lower-case l (or sometimes \ell) was still normal as well. These days, I rarely see that.

(Off-topic: “Small caps” is not the same as “lower case”.)